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NASA's Spitzer Finds Possible Comet Dust Around Dead Star

An artist's concept of a comet being torn to shreds around a dead star, or white dwarf, called G29-38. Image credit: NASA/JPL-Caltech.+ Full image and caption

NASA's Spitzer Space Telescope has spotted what may be comet dust
sprinkled around the white dwarf star G29-38, which died approximately
500 million years ago.

The findings suggest the dead star, which most likely consumed its inner
planets, is still orbited by a ring of surviving comets and possibly outer
planets. This is the first observational evidence that comets can outlive
their suns.

"Astronomers have known for decades that stars are born, have an extended
middle age, and then wither away or explode. Spitzer is helping us understand
how planetary systems evolve in tandem with their parent stars," said David
Leisawitz, NASA's Spitzer program scientist.

Astronomers believe white dwarfs are shrunken skeletons of stars that were
once similar to Earth's sun. As the stars aged over billions of years, they grew
brighter and eventually swelled in size to become red giants. Millions of years
later, the red giants shed their outer atmospheres, leaving behind white dwarfs.

If any planets did orbit in these systems, the red giants would have engulfed at
least the inner ones. Only distant outer planets and an orbiting icy outpost of
comets would have survived.

"The dust seen by Spitzer around G29-38 was probably generated relatively recently
when one such outlying comet may have been knocked into the inner region of the
system and ripped into dust shreds by the tidal forces of the star," said astronomer
William Reach of the Spitzer Science Center at the California Institute of
Technology in Pasadena, Calif.

Prior to the Spitzer findings, astronomers studying G29-38 noticed an unusual
and unknown source of infrared light. Spitzer, with its powerful infrared spectrometer,
was able to break this light apart, revealing its molecular makeup. These data told
astronomers the light was coming from the same types of dusty minerals found in
comets in our solar system.

"We detected a large quantity of very small, dirty silicate grains," said astronomer
Marc Kuchner of NASA's Goddard Space Flight Center, Greenbelt, Md. "The size of these
grains tells us they are probably from comets and not other planetary bodies."

In our own solar system, comets reside in the cold outer fringes in regions known as
the Kuiper Belt and Oort Cloud. Only when something disturbs their orbits, such as
another comet or an outer planet, do they begin periodic journeys into the sun's warmer
neighborhood. However, these trips to the tropics often end in destruction. Comets slowly
disintegrate as they pass close to the sun, or they crash into it. They also occasionally
crash into planets, as comet Shoemaker-Levy 9 did when it plunged into Jupiter.

Though the dust seen by Spitzer around the white dwarf is most likely the remains of
such a torn-up comet, there may be other explanations. One possibility is that a second
wave of planets formed long after the death of the star, leaving a dusty construction zone.

Kuchner presented his findings today at the 207th meeting of the American Astronomical
Society in Washington. The data were also published in the December 20, 2005, issue of
the Astrophysical Journal.